Method of fabricating medium-high temperature CO2 sorbents made of alkali metal promoted calcium aluminate carbonates

ABSTRACT

A dry material is synthesized by alkali metal (Li, Na and K) promoted calcium aluminate carbonates to obtain a CO 2  sorbent used at a temperature higher than 600 Celsius degrees (° C.). The key composition of the sorbents is 52˜69% of CaO, which is beneficial to capture CO 2  at 400˜800° C. A breakthrough result is achieved by using this sintering-resistant sorbent, which includes the features of 50% initial carbonation capacity and 20 folds CO 2  capturing performance maintained after 40˜60 hours. Besides, alkali bearing material provides good velocity in CO 2  capturing/releasing cycles; for example, by using Li and K series sorbents, 40 hours is required for 40 cycles only.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates to fabricating CO₂ sorbents; more particularly, relates to fabricating CO₂ sorbents with calcium aluminate carbonates promoted with alkali metals for capturing CO₂ at a medium-high temperature.

DESCRIPTION OF THE RELATED ARTS

The most effective way for reducing CO₂ is to capture CO₂ from a steady and enormous emission source, like coal power plant, petrochemical plant, cement plant, steel plant, etc. Two ways are used for capturing CO₂: one is a wet way and the other is a dry way. The wet way is usually called post-combustion, using a material like monoethanolamine (MEA), methylethanolamine (MDEA), diethonalamine (DEA), Triethonalamine (TEA), alkali solution, etc. to be used at a temperature between 100° C. and 150° C. The dry way has the follow categories: When being used at a temperature between room temperature and 400° C., sorbents for post-combustion capture are usually used, like carbon-based sorbents, supported-amine sorbents, zeolite sorbents, etc; and, when used at a temperature between 400° C. and 800° C., sorbents for pre-combustion and post-combustion capture are used, like CaO/CaCO₃, Li₄SiO₄, Li₂ZrO₃, etc.

The dry way can be used at a higher temperature, like more than 600° C.; and CaO is the most used. For sorbents used in the higher temperature, main concerns include reducing deterioration, improving capture capacity, etc. Yet, the key concern for the sorbents is stability.

Stability is directly related to capture capacity. It is because carbonation of CaO happened on capturing CO₂ will produce CaCO₃, which may likely be sintered. Hence, to shorten the process time for capturing CO₂ becomes the main concern. Under an environment having a steady temperature and a steady pressure, CO₂ capture is related to the following factors: inverse to ion size, diffusion coefficient and direct to square root of reaction time. On capturing CO₂ at a medium-high temperature, the outermost layer of ions of the sorbents will diffuse very fast and thus enhance reactions of capturing/releasing CO₂, Li₂CO₃, Na₂CO₃ and K₂CO₃ are not fit to be directly used for capturing CO₂ owing to their small capture amount ratios (<10 wt %) and their low decomposition temperatures (e.g. smaller than 400° C. for K₂CO₃). They may be fit to be used as promoters for making compound materials of medium-high CO₂ sorbents, like Li₄SiO₄ and Li₂ZrO₃; NaOH/CaO and Na₂ZrO₃; and K₂CO₃/Al₂O₃ (or K₂CO₃/CaO) and K₂Ti₂O₅. Yet, sorbents directly promoted with alkali metals to be used for CO₂ capture at a temperature higher than 600° C. are still not found. Hence, the prior arts do not fulfill all users' requests on actual use.

SUMMARY OF THE DISCLOSURE

The main purpose of the present disclosure is to fabricating CO₂ sorbents with calcium aluminate carbonates promoted with alkali metal, like Li, Na, K, etc., for capturing CO₂ at a medium-high temperature of 400˜800° C. with a fast capture velocity, a good stability and a high amount ratio of CO₂ captured.

To achieve the above purpose, the present disclosure is a method of fabricating medium-high temperature CO₂ sorbents made of alkali metal promoted calcium aluminate carbonates, comprising step of: (a) mixing a source of Ca ions and Al(NO₃)₃.9H₂O to obtain an acid solution, where the source of calcium ions is CaCl₂, Ca(NO₃)₂ or Ca(CH₃COO)₂.xH₂O and x is a value between 0 and 1; and where a mole ratio of the source of calcium ions to Al(NO₃)₃.9H₂O is between 1:1 and 30:1 in the acid solution; (b) mixing NaOH and a carbonate to obtain an alkali solution, where the alkali solution has 1.6 mole of NaOH; and where the carbonate is Li₂CO₃, Na₂CO₃ or K₂CO₃; (c) mixing the acid solution and the alkali solution to obtain a mixed solution and staying still the mixed solution; and (d) after staying still the mixed solution, filtering the mixed solution to remove impurities and drying mixed solution to be sintered in an oven to obtain a CO₂ sorbent, wherein, according to the carbonate used in step (b), the CO₂ sorbent thus obtained is Li—Ca—Al—CO₃, Na—Ca—Al—CO₃ or K—Ca—Al—CO₃. Accordingly, a novel method of fabricating medium-high temperature CO₂ sorbents made of alkali metal promoted calcium aluminate carbonates is obtained.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The present disclosure will be better understood from the following detailed description of the preferred embodiment according to the present disclosure, taken in conjunction with the accompanying drawings, in which

FIG. 1 is the flow view showing the preferred embodiment according to the present disclosure;

FIG. 2 is the view showing the TG curves for the preferred embodiment at different temperatures;

FIG. 3 is the view showing the TG curves of the different CO₂ sorbents;

FIG. 4 is the view showing the TG curves in one cycle;

FIG. 5 is the view showing the time periods spent for 40 cycles of capturing/releasing CO₂ and the amounts of CO₂ captured in the 40 cycles;

FIG. 6 is the view showing the characteristics of the different CO₂ sorbents; and

FIG. 7 is the view showing the stabilities and the characteristics of the different CO₂ sorbents after 40 cycles of capturing/releasing CO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description of the preferred embodiment is provided to understand the features and the structures of the present disclosure.

Please refer to FIG. 1, which is a flow view showing a preferred embodiment according to the present disclosure. As shown in the figure, the present disclosure is a method of fabricating medium-high temperature CO₂ sorbents made of alkali metal promoted calcium aluminate carbonates, where an alkaline oxide and Al₂O₃ are used as main components to be bond with a carbonate having ions of an alkali element, like lithium (Li), sodium (Na) or potassium (K), for a co-precipitation reaction to form a medium-high temperature CO₂ sorbent through sintering; and where, in a medium-high temperature gas flow, CO₂ is captured by the sorbents in fast reaction velocities with stable capture ratios and amounts. The present disclosure comprises the following steps:

(a) Preparing acid solution 11: A source of calcium (Ca) ions and Al(NO₃)₃.9H₂O are mixed to obtain an acid solution, where the source of calcium ions is CaCl₂, Ca(NO₃)₂ or Ca(CH₃COO)₂.xH₂O and x is a value between 0 and 1; and where a mole ratio of the Ca ions source to the Al(NO₃)₃.9H₂O is between 1:1 and 30:1 in the acid solution.

(b) Preparing alkali solution 12: NaOH and a carbonate are mixed to obtain an alkali solution, where the alkali solution has 1.6 mole of NaOH; and where the carbonate is Li₂CO₃, Na₂CO₃ or K₂CO₃.

(c) Mixing solutions 13: The acid solution and the alkali solution are mixed by stirring for 24 to 36 minutes to obtain a mixed solution; and, then, the mixed solution is stayed still for 24 hours (hrs).

(d) Drying and filtering 14: After the mixed solution is stayed still, the mixed solution is filtered to remove impurities and is dried to be sintered in an oven at 600 Celsius degrees (° C.) for obtaining a CO₂ sorbent, where, according to the carbonate used in step (b), the CO₂ sorbent thus obtained is Li—Ca—Al—CO₃, Na—Ca—Al—CO₃ or K—Ca—Al—CO₃.

If Li₂CO₃ is used in step (b) to be mixed with 1.6 mole of NaOH, a CO₂ sorbent of Li—Ca—Al—CO₃ is obtained in step (d); if Na₂CO₃ is used, Na—Ca—Al—CO₃ is obtained; and, if K₂CO₃ is used, K—Ca—Al—CO₃ is obtained.

On using the present disclosure, the CO₂ sorbent of Li—Ca—Al—CO₃, Na—Ca—Al—CO₃ or K—Ca—Al—CO₃ captures CO₂ at a temperature between 400° C. and 800° C. After absorbing CO₂ to obtain CaCO₃, CO₂ density can be reduced by heating to recycle CaCO₃ into CaO for regaining capture activity. Such a cycle is expressed with the following two chemical formulas: Capturing: CaO+CO₂→CaCO₃ Releasing: CaCO₃→CaO+CO₂

Please refer to FIG. 2 and FIG. 6, which are a view showing TG curves for a preferred embodiment at different temperatures; and a view showing characteristics of different CO₂ sorbents. As shown in the figures, each single cycle processed by CO₂ sorbents fabricated according to the present disclosure includes steps of capturing and releasing CO₂. The ratio of CO₂ captured during a few cycles of capturing/releasing CO₂ shows stability of the CO₂ sorbents. At 750° C., thermogravimetric (TG) analyses are processed for 30 cycles of capturing/releasing CO₂. Curves 21,22,23 in FIG. 2 show initial amount ratios of 56%, 55% and 49%. The amount ratios remain 53%, 45% and 47% after 30 cycles of capturing/releasing CO₂, which show stability of the CO₂ sorbents.

When the ratio of Ca to Al is bigger than 7, complex crystalline phase may be produced with high cost and quality may become hard to control. Hence, in FIG. 6, an acetic acid is used to fabricate the CO₂ sorbents having a ratio of Ca to Al as 7 for measuring their characteristics, including surface areas, pore characteristics and other chemical characteristics. The CaO wt % is figured out by a factor of CaO/Ca, i.e. CaO=Ca×1.4. Therein, the average granular size is 10˜43 μm; the surface area is 14.2˜17.3 m²/g; the pore size is 0.02 cc/g, which is a micropore or a mesopore; and, the average pore diameter is 5.3˜20.5 nm.

The CaO contents in the CO₂ sorbents have great impacts on CO₂ capture capacity. The CO₂ sorbents are dissolved in acid solutions for measuring elements of Ca, Al, etc. with inductively coupled plasma (ICP) for figuring out CaO contents. For Li, Na and K series CO₂ sorbents, the CaO contents are 51.9%, 68.9% and 56.5%, respectively. Therein, the ratios of Li, Na and K contained are 0.02% for Li series, 0.8% for Na series and 0.04% for K series.

Please refer to FIG. 3, which is a view showing TG curves of different CO₂ sorbents. As shown in the figure, CO₂ sorbents fabricated according to the present disclosure, which contains Li, Na and K ions, are used to capture CO₂ from a room temperature to 950° C. for obtaining TG curves. As shown in TG curves 31,32,33 for the CO₂ sorbents fabricated according to the present disclosure, CO₂ is captured at a temperature between 400° C. and 800° C. with initial capture amount ratios higher than 50 wt %, whose performances are obviously better than traditional sorbents of CaO and CaCO₃ as shown in other TG curves 34,35. Besides, some more sorbents of Li₂CO₃, Na₂CO₃ and K₂CO₃ have initial capture amount ratios lower than 10 wt %. Hence, the CO₂ sorbents fabricated according to the present disclosure are better than other sorbents of CaO, CaCO₃, Li₂CO₃, Na₂CO₃ and K₂CO₃. Moreover, the Li series or K series sorbents fabricated according to the present disclosure have higher CO₂ capture velocity (slope) than the Na series sorbent at a temperature higher than 600° C., not to mention the other sorbents.

Please refer to FIG. 4, which is a view showing TG curves in one cycle. As shown in the figure, Li series, Na series and K series CO₂ sorbents fabricated according to the present disclosure are used to process a single cycle of capturing/releasing CO₂ at 750° C. As results show, capture rates of the sorbents do not have big different in between. But, the Li series and K series sorbents have better performance on releasing CO₂. The capture rates of the sorbents in a single cycle are 1.0 hour (hr) for Li series sorbent, 1.5 hrs for Na series sorbent and 1.0 hr for K series sorbent.

Please refer to FIG. 5, which is a view showing time periods spent for 40 cycles of capturing/releasing CO₂ and amounts of CO₂ captured in the 40 cycles. As shown in the figure, 40 cycles of capturing/releasing CO₂ are processed to sorbents fabricated according to the present disclosure for acquiring best reaction times for capturing the most CO₂. As referring to FIG. 4, 40 cycles for the sorbents are 1.0 hr for Li series sorbent, 1.5 hrs for Na series sorbent and 1.0 hr for K series sorbent. The amount ratios of CO₂ captured are 54˜51 wt % for Li series sorbent, 53˜49 wt % for Na series sorbent and 53˜49 wt % for K series sorbent. Conclusively, regarding capture amount, Li series sorbent has the best performance, followed by Na series sorbent and K series sorbent; regarding reaction time, Li series sorbent has the best performance; and, in the end, the Li series sorbent fabricated according to the present disclosure is the best sorbent.

Please refer to FIG. 7, which is a view showing stabilities and characteristics of different CO₂ sorbents after 40 cycles of capturing/releasing CO₂. as shown in the figure, CO₂ sorbents fabricated according to the present disclosure and traditional sorbents of CaO, limestone and Li₄SiO₄ are used to obtain their stabilities and characteristics after 40 cycles of capturing/releasing CO₂, where CO₂ is captured in an environment having a 100% CO₂ concentration and a temperature of 650˜750° C.; and where CO₂ is released in an environment having a 100% N₂ density ratio and a temperature of 650750° C. A result is as follows:

(1) A single cycle of capturing/releasing CO₂ spends 1.0˜1.5 hrs and, so, 40 cycles spend 40˜60 hrs, where the time for the cycle includes time for capturing CO₂ and time for releasing CO₂ in the cycle.

(2) Stable amount ratios of CO₂ captured (gCO₂/g sorbent) for 1˜40 cycles are 49˜54 wt % for the sorbent fabricated according to the present disclosure; 67˜54 wt % for CaO; 44˜36 wt % for limestone; and 22˜32 wt % for Li₄SiO₄, where each of the ratios is a ratio of increased weight after capturing CO₂ in current cycle to original weight after releasing CO₂ in previous cycle.

(3) The sorbents fabricated according to the present disclosure have high R40 values bigger than 94%, which reach the highest international standard (85˜90%) and are almost equal to Li₄SiO₄'s performance (R40=94%). In addition, after 40˜60 hrs of using, the sorbents still have stable performances obviously higher than CaO, limestone and Li₄SiO₄. Therein, R40 is the amount of CO₂ captured after 40 cycles of capturing/releasing CO₂.

(4) After 40 cycles of capturing/releasing CO₂, the weights of CO₂ captured by the sorbents fabricated according to the present disclosure is 20 times to the original weights of the sorbents used.

(5) Total amount of CO₂ captured is related to the sorbent used and the time spent for capturing/releasing CO₂. This characteristic is expressed with a unit of mgCO₂/mg sorbent.h, where the sorbents fabricated according to the present disclosure have a value of 0.52 and is better than the other values, like 0.34.

Hence, the present disclosure fabricates medium-high temperature CO₂ sorbents with calcium aluminate carbonate promoted with alkali metal, where the sorbents contains 52˜69% of CaO to capture CO₂ at a temperature of 400˜800° C. with initial capture ratios higher than 50 wt %. Furthermore, the sorbents combine CaO and Al₂O₃ to maintain good performances for 40 to 60 hrs with weights of CO₂ captured 20 times to original weights of the sorbents, where the Na series sorbent spends 50 hrs for 40 cycles of capturing/releasing CO₂ and the Li and K series sorbents spend 40 hrs for 40 cycles only.

To sum up, the present disclosure is a method of fabricating medium-high temperature CO₂ sorbents made of alkali metal promoted calcium aluminate carbonates, where calcium aluminate carbonates promoted with alkali metals are fabricated into CO₂ sorbents having capture ratios higher than 50 wt % during a time period more than 40 hrs and having fast capture velocities for spending just 40 hrs for 40 cycles of capturing/releasing CO₂.

The preferred embodiment herein disclosed is not intended to unnecessarily limit the scope of the disclosure. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present disclosure. 

What is claimed is:
 1. A method of fabricating medium-high temperature CO₂ sorbents made of alkali metal promoted calcium aluminate carbonates, comprising steps of: (a) mixing a source of Ca ions and Al(NO₃)₃.9H₂O to obtain an acid solution, wherein said source of calcium ions is selected from a group consisting of CaCl₂, Ca(NO₃)₂ and Ca(CH₃COO)₂.xH₂O and x is a value between 0 and 1; and wherein a mole ratio of said source of calcium ions to said Al(NO₃)₃.9H₂O is between 1:1 and 30:1 in said acid solution; (b) mixing NaOH and a carbonate to obtain an alkali solution, wherein said alkali solution has 1.6 mole of NaOH; and wherein said carbonate is selected from a group consisting of Li₂CO₃, Na₂CO₃ and K₂CO₃; (c) mixing said acid solution and said alkali solution to obtain a mixed solution and staying still said mixed solution; and (d) after staying still said mixed solution, filtering said mixed solution to remove impurities and drying said mixed solution to be sintered in an oven to obtain a CO₂ sorbent, wherein, according to said carbonate used in step (b), said CO₂ sorbent thus obtained is selected from a group consisting of Li—Ca—Al—CO₃, Na—Ca—Al—CO₃ and K—Ca—Al—CO₃.
 2. The method according to claim 1, wherein said CO₂ sorbent captures CO₂ at a temperature between 400° C. and 800° C.; wherein said CO₂ sorbent has an initial CO₂ capture ratio bigger than 50 wt %; and wherein said CO₂ sorbent captures CO₂ for a time period between 40 hours (hrs) and 60 hrs.
 3. The method according to claim 1, wherein Li—Ca—Al—CO₃ spends 40 hrs to process 40 cycles of CO₂ capture with a capture ratio higher than 51 wt %; wherein Na—Ca—Al—CO₃ spends 60 hrs to process 40 cycles of CO₂ capture with a capture ratio higher than 49 wt %; and wherein K—Ca—Al—CO₃ spends 40 hrs to process 40 cycles of CO₂ capture with a capture ratio higher than 49 wt %.
 4. The method according to claim 1, wherein said CO₂ sorbent has an average amount ratio of CaO between 52% and 69%. 